Former FBI Director James Comey in his first exclusive interview since he was fired last May said that Donald Trump was "morally unfit" to be the President of the US.

In the exclusive ABC News interview on Sunday night, Comey dismissed claims made by some that Trump was medically unfit to hold office.

"I often hear people talk about it. I don't buy this stuff about him being mentally incompetent or early stages of dementia. He strikes me as a person of above average intelligence who's tracking conversations and knows what's going on," Comey said.

"A person who treats women like they're pieces of meat, who lies constantly about matters big and small and insists the American people believe it, that person's not fit to be president of the United States, on moral grounds," Comey added.

The interview comes ahead of the release of his tell-all book "A Higher Loyalty: Truth, Lies, and Leadership" on Tuesday.

Also a lawyer, Comey told ABC News that he was standing in the middle of the Los Angeles Federal Bureau of Investigation field office on May 9, 2017, thanking employees and the building's support staff for their hard work, when he saw the televisions on the wall flash "Comey Resigns".

"One of the many great things about the FBI is we have some hilarious pranksters, and so I thought it was a scam by someone on my staff..."

As the TVs in the bureau started to show the news on other networks, he saw some were displaying the words "Comey Fired".

"The audience could see my face change," he said.

Comey said his reaction to the news was disbelief. "That's crazy... How could that be?"

He received a call from then-Secretary of Homeland Security John Kelly who was "very upset" with the news and was thinking about quitting too.

Comey said he urged Kelly to remain in the administration.

The former FBI Director said that he was "actually nervous" to meet Trump in person.

"I'm about to meet a person who doesn't know me, who's just been elected president of the United States... And I'm about to talk to him about allegations that he was involved with prostitutes in Moscow and that the Russians taped it and have leverage over him.

"He had impressively coiffed hair, his tie was too long as it always is, he looked slightly orange up close with small white half-moons under his eyes, which I assume are from tanning goggles," he said.

When asked if the Russians had something on Trump, Comey said: "I think it's possible. I don't know... These are more words I never thought I'd utter about a president of the United States, but it's possible."

Comey told ABC News that he knew the investigation into former Secretary of State Hillary Clinton's use of a private email server was going to present a "no-win situation" for him.

Comey revealed that while he did not vote in the 2016 election saying that as the FBI director he was "trying to be outside of politics" and there were a lot of Clinton supporters in his family.